HCCI is a known process for fueling a diesel engine in a manner that creates a substantially homogeneous air-fuel charge inside an engine cylinder during a compression upstroke of an engine cycle. After a desired quantity of fuel for the charge has been injected into the cylinder to create a substantially homogeneous air-fuel mixture, the increasing compression of the charge by the upstroking piston creates sufficiently large pressure to cause auto-ignition of the charge. In other words, the HCCI mode of operation of a diesel engine may be said to comprise 1) injecting a desired amount of fuel into a cylinder at an appropriate time during the compression upstroke so that the injected fuel mixes with charge air that has entered the cylinder during the preceding intake downstroke and early portion of the compression upstroke in a manner that forms a substantially homogeneous mixture within the cylinder, and then 2) increasingly compressing the mixture to the point of auto-ignition near or at top dead center (TDC). Auto-ignition may occur as the substantially simultaneous spontaneous combustion of vaporized fuel at various locations within the mixture. No additional fuel is injected after auto-ignition.
One of the attributes of HCCI is that relatively lean, or dilute, mixtures can be combusted, keeping the combustion temperatures relatively low. By avoiding the creation of relatively higher combustion temperatures, HCCI can yield significant reductions in the generation of NOx, an undesired constituent of engine exhaust gas.
Another attribute of HCCI is that auto-ignition of a substantially homogeneous air-fuel charge generates more complete combustion and consequently relatively less soot in engine exhaust.
The potential benefit of HCCI on reducing tailpipe emissions is therefore rather significant, and consequently HCCI is a subject of active investigation and development by many scientists and engineers in the engine research and design community.
One aspect of HCCI seems to impose a limit on the extent to which it can provide drastically reduced tailpipe emissions of soot and NOx. At higher engine speeds and larger engine loads, the rate of combustion is difficult to control. Consequently, known engine control strategies may utilize HCCI only at relatively lower speeds and smaller engine loads. At higher speeds and/or larger loads, the engine is fueled so that the fuel combusts by conventional diesel (CD) combustion upon being injected into charge air that has been compressed within a cylinder to a pressure that is sufficiently great to cause the fuel to combust as it is being injected.
With the advent of processor-controlled fuel injection systems capable of controlling fuel injection with precision that allows fuel to be injected at different injection pressures, at different times, and for different durations during an engine cycle over the full range of engine operation, a diesel engine becomes capable of both CD combustion and HCCI combustion.
As will be explained by later description, the present invention takes advantage of the capabilities of those fuel injection and processing systems to control fuel injections in different ways depending on certain aspects of engine operation. Exactly how any particular fuel injection system will be controlled by an associated processing system in any given engine will depend on specifics of the engine, the fuel injection system, and the processing system.
Because a diesel engine that powers a motor vehicle runs at different speeds and loads depending on various inputs to the vehicle and engine that influence engine operation, fueling requirements change as speed and load change. An associated processing system processes data indicative of parameters such as engine speed and engine load to develop control data for setting desired engine fueling for particular operating conditions that will assure proper control of the fuel injection system for various combinations of engine speed and engine load.
Pending U.S. patent application Ser. No. 10/809,254 discloses a diesel engine and associated processor-controlled fuel injection system that processes certain data to select one of plural fueling modes for operating the engine. When a result of the processing selects a first fueling mode (HCCI mode), the engine is fueled during an engine cycle to create a substantially homogeneous air-fuel charge within one or more combustion chambers. That charge is compressed to combust by auto-ignition, with no more fuel being introduced after auto-ignition. When a result of the processing selects a second fueling mode (HCCI-CD mode), the engine is fueled during an engine cycle to create a substantially homogeneous air-fuel charge within the one or more combustion chambers. That charge is compressed to combust by auto-ignition (HCCI), after which more fuel is introduced into the one or more combustion chambers to provide additional combustion (CD). That engine utilizes HCCI combustion at relatively smaller loads and relatively smaller speeds and what is referred to as HCCI-CD combustion at relatively larger loads and relatively higher speeds. The present invention relates to an engine, system, and method for enhancing the use of HCCI combustion in a diesel engine toward objectives that include reducing the generation of undesired constituents in engine exhaust, especially soot and NOx, and improving thermal efficiency. The invention is embodied in the fuel injection control strategy, a strategy that is programmed in an associated processing system.
According to principles of the present invention, the utilization of HCCI combustion occurs in a different manner from that described in U.S. patent application Ser. No. 10/809,254. The present invention comprises three distinct modes of engine operation: 1) an HCCI+RVT mode; 2) an HCCI+VVT mode; and 3) a CD+RVT mode. Each of these modes will be explained in detail hereinafter. The HCCI+RVT mode is utilized at relatively smaller loads and relatively lower speeds. The HCCI+VVT mode is utilized at relatively larger loads than those of the HCCI+RVT mode and at relatively higher speeds than those of the HCCI+RVT mode. The CD+RVT mode is utilized at still relatively larger loads than those of the HCCI+VVT mode and at still relatively higher speeds than those of the HCCI+VVT mode.
The HCCI+VVT mode enables the benefits of HCCI to be obtained in a portion of the engine operating range between the portion of the range where HCCI+RVT is exclusively used and the portion of the range where CD+RVT is exclusively used.
One generic aspect of the present invention relates to a method of operating a compression ignition engine. Certain data is processed by an engine control system to select one of plural fueling modes for operating the engine.
When the result of the processing selects a first fueling mode (HCCI+RVT), one or more combustion chambers are fueled to create, within each such combustion chamber during a corresponding engine cycle, a substantially homogeneous air-fuel charge that is compressed to auto-ignition without the introduction of any additional fuel after auto-ignition during that corresponding engine cycle.
When the result of the processing selects a second fueling mode (HCCI+VVT), the effective compression ratio for each such combustion chamber is reduced from the effective compression ratio used during the first fueling mode (HCCI+RVT), and each such combustion chamber is fueled to create a substantially homogeneous air-fuel charge that is compressed to auto-ignition without the introduction of any additional fuel after auto-ignition during that corresponding engine cycle.
When the result of the processing selects a third fueling mode (CD+RVT), each such combustion chamber is fueled by introducing fuel at a time during the engine cycle when air in the corresponding combustion chamber has been compressed sufficiently to cause the fuel to combust as it is being introduced.
Another generic aspect of the invention relates to a compression ignition engine that operates according to the method just described.
Still another generic aspect of the invention relates to a compression ignition engine comprising a control system for processing data, one or more combustion chambers, and a fueling system for injecting fuel into the one or more combustion chambers.
The control system controls the fueling system to extend the load/speed range over which HCCI combustion occurs by a) fueling one or more combustion chambers over a relatively lower speed/load range to create a substantially homogeneous air-fuel charge within each such combustion chamber during a corresponding engine cycle and compressing each charge to auto-ignition without introducing any additional fuel after auto-ignition during that corresponding engine cycle, and b) decreasing the effective compression ratio of each such combustion chamber and fueling each such combustion chamber with increased fueling over a relatively higher speed/load range to create a substantially homogeneous air-fuel charge within each such combustion chamber during a corresponding engine cycle and compressing each charge to auto-ignition without introducing any additional fuel after auto-ignition during that corresponding engine cycle.
Another generic aspect relates to a compression ignition engine that operates according to the method just described.
In disclosed embodiment of the invention, the data that is processed to select the particular mode comprises engine speed data and engine load data. Injection pressure, duration, and timing may differ from mode to mode. Data for the various modes are contained in maps in the engine control system.
The foregoing, along with further features and advantages of the invention, will be seen in the following disclosure of a presently preferred embodiment of the invention depicting the best mode contemplated at this time for carrying out the invention. This specification includes drawings, now briefly described as follows.